1. Field of the Invention
This invention is directed to analog to digital converters (ADC) and down converters for bandpass signals. More specifically, the invention is directed to generating the analog Hilbert transform of the bandpass signal by means of a quadrature (90-degree) hybrid.
2. Description of the Background
Sampling broadband signals with high relatively bandwidths is a complicated task. One approach is to consider the signal as a baseband signal and sample the signal at least at the Nyquist sampling rate. For a relative bandwidth of 100%, this implies a sampling rate at least 3 times the signal bandwidth. A second approach, known as undersampling or bandpass sampling, consists of directly sampling the RF/IF signal using a sampling frequency such that the frequency band of interest is entirely contained within the limits of one “Nyquist zone”. In general, this requires a higher sampling frequency than the method proposed and is possible (with no additional hardware) only for a small enough relative bandwidth. A third approach would be to downconvert the signal, but once again, additional hardware is required and a good linearity can be achieved only if the relative bandwidth is small.
In some existing methods to sample broadband signals a Local Oscillator (LO) signal is inputted into a 90-degree hybrid whose outputs are then mixed with the input RF/IF signal, e.g. see (FIG. 4 of U.S. Pat. No. 7,356,103).
Another approach is to digitize directly the bandpass signal as if it were a baseband signal, using a sampling rate faster than the highest frequency. No quadrature hybrid is used in this method. For example, U.S. Pat. No. 4,982,193 provides a similar method to digitize at the carrier frequency, or preferably, one of its harmonics. Also, U.S. Pat. Nos. 8,279,100 and 7,692,570 each provide systems to directly digitize an RF signal at sampling rates commensurable with the signal bandwidth by means of a plurality of ADCs.
As another example of the existing methods, U.S. Pat. No. 6,466,150 provides an ADC that is suited for digitizing the phase of the signal. The method exclusively focuses on digitizing the phase only, with application to PM or FM signals. Moreover, the digitizer operates on baseband (downconverted) signals and does not utilize a full-wave rectifier, leading to more comparators (more power consumption) than the proposed method.
US Patent Application No. 2013/0069812 discloses an alternative method to digitize bandpass signals located in any Nyquist zone by means of a time-interleaved analog-to-digital converter. It applies to time-interleaved ADC is used and the Hilbert transform is applied in the digital domain, so the analog input is not a complex signal. This method suffers from the need of a calibration method inherent to its time-interleaved nature.
U.S. Pat. No. 8,482,445 also applies to time-interleaved ADCs, and hence it is also hindered by the need of complex calibration methods. The calibration method disclosed is a time-interleaved ADC by the injection of a probe tone at a different Nyquist zone. The Hilbert transform is applied in the digital domain.
U.S. Pat. No. 6,301,312 is directed to phase-locked loops. It employs a conventional IQ demodulation where the IF signal is mixed with the in-phase and quadrature versions of a local oscillator. This system uses mixers which can lead to mixer insertion loss, typically about 20 dB.